Organic compounds in an aqueous stream can be oxidized to carbon dioxide and water using several methods. One well know method is Wet Air Oxidation (WAO) or the Zimmerman process (U.S. Pat. No. 2,665,249). According to this process, an organic material and an oxidizing agent, frequently air or pure oxygen, are heated in a pressurized reactor so that the reaction temperature remains below the critical temperature of water (about 374.degree. C.) and the pressure is in the range of about 1500 to 2500 psi. At these temperatures and pressures both a liquid and a gas phase are present. Residence times of 0.5 to 1.0 hours result in oxidation of 70% to 95% of the organic compounds in the waste stream.
If more complete oxidation of the organic compounds is sought, the oxidation may be carried out under supercritical conditions for the aqueous stream (typically a temperature greater than 374.degree. C. and a pressure greater than 3200 psia). This process, known as Supercritical Water Oxidation (SCWO), typically requires residence times of a few seconds to a few minutes and can result in the oxidation of more than 99% of the organic compounds present.
Water under supercritical conditions forms a single fluid phase having quite different characteristics from water in a liquid-gas two phase system of the type which exists under subcritical conditions. For example, subcritical water is a poor solvent for non-polar materials (including many organic materials) and a good solvent for polar materials (including many inorganic materials). However, at and above the critical point of water many organic compounds become readily soluble in water and many inorganic compounds become insoluble.
For example, at 25.degree. C. benzene is sparingly soluble in water (0.07 weight percent). However, under supercritical conditions, benzene and water are completely miscible in all proportions (Connolly, J., "Solubility of Hydrocarbons in Water Near the Critical Solution Temperature", J. Chem Eng. Data 11(1), 13 (1966)).
Sodium chloride (NaCl) has a solubility of about 37 weight percent (370,000 ppm) under subcritical conditions at 300.degree. C. and about 120 ppm under supercritical conditions of 550.degree. C. and 25 MPa (Pitzer, K. S. and R. T. Pabalan, "Thermodynamics of NaCl in Steam", Gechim. Cosmochim. Acta 50, 1445 (1986)).
Calcium chloride (CaCl.sub.2) has a maximum solubility of 70 weight percent at subcritical temperatures and 3 ppm at 500.degree. C. and 25 MPa (Martynova, O. I., "Solubility of Inorganic Compounds in Subcritical and Supercritical Water", in High Temperature, High Pressure Electrochemistry in Aqueous Solutions, D. de G. Jones and R. W. Staehle, eds. Houston: National Association of Corrosion Engineers, (1976))
Oxygen is also completely miscible with water under supercritical conditions (Japas, M. L., and E. U. Franck, "High Pressure Phase Equilibria and PVT Data of the Water--Oxygen System Including Water-Air to 673.degree. K. and 250 MPa", Ber. Bunsenges Phys. Chem 89, 1268 (1985)).
The fact that oxygen and many organics are completely miscible with water under supercritical conditions means that they come into intimate contact in a single phase contributing to very rapid oxidation reactions.
However, the insolubility of many inorganic compounds under supercritical conditions results in a serious scaling problem, with inorganic precipitates fouling surfaces and valves inside, and downstream of, the reactor, which is one of the problems to which this patent is directed.
The environment in an SCWO reactor can also be a reactive and corrosive one because of the presence of oxygen and the relatively high temperatures employed. Organic chlorine compounds produce chloride ions which are very reactive, and corrosive to metal surfaces, at supercritical temperatures.
U.S. Pat. No. 2,944,396 to Barton and Zimmerman et al. describes "vapour phase" oxidation of organic compounds in an aqueous stream. It describes an improvement to the wet air oxidation process of Zimmerman wherein a second oxidation stage is added. The effluent vapours from the wet air oxidation process are oxidized in a second reactor under conditions of 842.degree. F. to 1034.degree. F. (column 5, lines 40-53) and pressures from 800 to 6,500 psi, which pressure range encompasses both subcritical and supercritical conditions. The result is substantially complete combustion of all organics (column 5, line 60).
More recently, supercritical water oxidation processes have been disclosed which directly treat organic compounds in an aqueous stream without a prior wet air oxidation step. U.S. Pat. No. 4,292,953 to Dickinson discloses the supercritical water oxidation of a carboniferous fuel to produce thermal, mechanical or electrical energy. Dickinson notes that if the salt concentration is too high, it can result in scaling of the reactor or scaling or plugging in down stream heat exchange equipment (column 6, lines 33-47).
U.S. Pat. No. 4,338,199 (Modell No. 1) and U.S. Pat. No. 4,543,190 (Modell No. 2) disclose the use of supercritical water oxidation to oxidize, and thereby destroy, toxic organic compounds and to produce useful energy.
According to the disclosure of these patents, the insolubility of many inorganic salts under supercritical conditions means that supercritical water oxidation can be used to desalinate sea water and brine. If the aqueous stream containing the organics is sea water or brine, under supercritical conditions salt precipitates out of the single fluid phase almost immediately, thus enabling desalination in a rapid and continuous process (Modell No. 1, column 2, lines 58-63). The patents note that inorganic material may tend to build up on the walls of the reactor causing hot spots with subsequent destruction of the reactor walls (Modell No. 1, column, lines 8-23).
The scaling problem resulting from the insolubility of inorganic compounds at supercritical conditions has been a major impediment to the commercialization of supercritical water oxidation. Many waste streams containing organic compounds also contain inorganic salts in concentrations sufficient to cause severe scaling under supercritical conditions resulting in frequent reactor shutdowns for descaling.
A number of attempts have been made to solve the scaling problem. U.S. Pat. No. 4,822,497 (Hong et al.) discloses a method of conducting supercritical water oxidation wherein the reactor has a supercritical temperature zone in the upper region of the reactor and a lower temperature zone in the lower region of the reactor which has a liquid phase. The supercritical water oxidation occurs in the upper region. Precipitates and other solids from the oxidized supercritical temperature zone are transferred to the lower temperature zone so as to produce a solution or slurry. The solution or slurry is then removed from the reactor.
U.S. Pat. No. 5,100,560 (Huang) discloses an alternate method for dealing with the scaling problem in the reactor. According to this disclosure, the reactor once again has a supercritical temperature zone and a lower temperature zone. At least a portion of the inner wall of the pressure vessel bounding the supercritical temperature zone is scraped so as to dislodge at least a substantial portion of any solids which may be deposited thereon.
PCT published application PCT/US92/05320 (Modell No. 3) describes a method for oxidizing organic compounds under supercritical conditions in an elongate tube reactor. The reaction mixture of organic material is passed through the reactor at a velocity sufficient to minimize settling of a substantial portion of the inorganic materials present in the reaction mixture or formed under supercritical conditions. The outlet end of the reactor is cooled to rapidly form a two phase system in the reaction mixture. Inorganic salts substantially redissolve in the liquid phase of the cooled reaction mixture to minimize scaling problems.
The methods describe above all have limitations and none are reported in commercial use at this time. Barton et al discloses a process wherein essentially no inorganic materials are present in the effluent vapour being oxidized under supercritical conditions. Hong et al. discloses operating a reactor to have both a supercritical zone and a subcritical zone. Hong et al does not contain any example to show that both zones could be maintained in an operating reactor. Huang discloses scraping the inner surface of the supercritical zone of the reactor of Hong et al to prevent scale build-up. However, Huang discloses that scale build-up does occur and the mechanical action disclosed in Huang would be difficult to operate in practice and would result in a decreased lifetime of the reactor. Modell No. 3 describes high velocity throughput to achieve some reduction in reactor scaling and supplementary descaling, for example by brushes, high velocity sprays and the use of filters.
Accordingly, while supercritical water oxidation has considerable potential for the treatment of many types of effluent, no solution has yet been developed to the scaling problem which arises from the insolubility of inorganic materials in the supercritical region.